Coalescence Filtration Research Articles

Coalescence performance of Janus filter for the removal of oil mist

The wettability has an important effect on coalescence performance of filter. In this paper, we utilized a commercial glass fiber filter as the substrate to prepared the Janus filter through dip-coating and electrospraying. The Janus filter exhibited a remarkable reduction in pressure drop, with only 2.8 kPa of wet pressure drop compared to 6.1 kPa for the original filter and 5.4 kPa for the oleophilic filter. Notably, the coating depth played a critical role in enhancing the coalescence performance of the Janus filter. Furthermore, when placing the Janus filter in reverse direction, its quality factor increased by 2.87 times, highlighting that orientation is an essential factor for coalescence performance of Janus filters. The results indicate great potential in the application of Janus filters for coalescence filtration.

High-fidelity 3D simulation of dust-loading behavior and clogging process of coarse nonwoven filter media considering collision effect

The filter lifetime of fibrous media is important for the energy-saving application of air filters and efficient delivery of clean air. The clogging process significantly affects the dust-loading behavior of fibrous filter media. Therefore, an effective methodology for determining the deposition of dust particles inside filter structures is necessary. This study conducted numerical dust loading simulations on a three-dimensional (3D) X-ray tomographic structure of coarse nonwoven filter media. Spherical particles with a density of 1808 kg/m3 were used in the simulations instead of plate-like dust particles with a higher silica density. A high-fidelity simulation model was successfully developed by considering collision effects. Key factors were determined, including the Hamaker constant for the adhesion energy, restitution coefficient for the particle kinetic energy, and sliding motion. Remarkably, the deviation between the simulated and experimental dust holding capacity was less than 1 %. The developed simulation model enabled a quantitative evaluation of the clogging process and 3D in-situ evolution of particle deposition and pore structures. It was observed that 95.6 % of the maximum deposited dust mass within the fibrous structure was captured at the clogging point. Several computational methods yielded accurate pore size evaluations and clear 3D pore structure visualizations. The developed methodology for high-fidelity simulation and 3D analysis can be further applied to other two-phase filtration and separation fields, including aerosol filtration, coalescence filtration, membrane fouling, micro/nanoplastic capture, and oil/water separation.

Development of an eco-friendly mobile plant for car wash wastewater recovery

The global increase in the number of vehicles has a negative impact on the sustainable development due to the increased energy consumption, noise, and air pollution, as well as the increased water consumption used for car wash. The car wash is one of the main after-sale services in the automobile sector. If car wash wastewater (CWW) could be reused, fresh water customarily used in car washing could be preserved. To achieve this goal, it is necessary to implement ?on-site? wastewater treatment systems (i.e. at car wash stations). In this research, a novel pilot plant is developed for ?on site? treatment of CWW. This pilot plant presents combined methods. It consists of three innovative modules: a multipurpose reactor (for coagulation/flotation, aeration/oxidation), deep bed filter (for filtration), and a two-stage bed coalescer (for coalescence filtration). In all these units packing materials are very light (expanded polystyrene (EPS) and polyurethane (PU)) and therefore energy consumption is significantly low. The pilot plant has a simple and light construction, so that it can be easily moved. It has shown very high COD reduction efficiency as well as removal efficiencies of suspended solids, and oil/grease, thus exhibiting high potentials for water recovery at car wash stations.

Aerogel-glass fiber composite filter media for effective separation of emulsified water droplets from diesel fuel

This work evaluates several mechanisms of separation of surfactant-stabilized emulsified water droplets from diesel fuel using high surface area (50–370 m2/g) and high porosity (>90 %) filter media fabricated by combining aerogels and glass fiber mats. Specifically, high surface area, porous gels of syndiotactic polystyrene (sPS) and silica are developed inside glass fiber mats via dip coating in corresponding sol followed by sol–gel transition. The resultant materials are supercritically dried to obtain aerogel-coated separation media. The abundant meso- and macropores of the aerogel network and the different surface energy of sPS and silica produce cooperative functioning of the mechanisms of size exclusion, coalescence filtration, surfactant adsorption, and volumetric water absorption for removal of emulsified water droplets from ultralow sulfur diesel fuel under continuous flow conditions. The role of each separation mechanism is elucidated in this paper. The results reveal high water separation efficiency of ∼92 % compared to ∼30 % for the glass fiber mats.

Toward formulating coalescence filtration: Characterizing wetting saturation via centrifugal force

Coalescence filtration is the removal of dispersed droplets from a gas or from an immiscible liquid using a fibrous filter. Coalescing filters operate under a partially-saturated condition where some of the filter pores are filled with accumulated droplets. To date, there exists no theory that can predict the filtration efficiency of a coalescing filter and this is due to the complicated coupling between the aerodynamic field inside the filter and the capillarity of the fibers. This paper presents a new approach to study coalescence filtration by replacing the aerodynamic field inside the filter with a centrifugal force field and to thereby decouple the role of fiber properties from that of the airflow in fluid accumulation in a filter. This paper is only the first step towards developing the above mathematical theory for coalescence filtration. In the current study, we use numerical simulation and experiment to compare desaturation of a liquid-saturated fibrous media via centrifugal force (new) and via compressed air (traditional). For the simulations, we used the volume-of-fluid (VOF) method implemented in ANSYS software, and for the experiments, we used a Porometer for the pressure-driven desaturation, and a custom-made setup inside a swing-bucket centrifuge for the centrifugal desaturation. The experiments were conducted using a nonwoven fabric infused with mineral oil. Pressure-driven and centrifugal desaturation processes were compared with one another, and the advantages of the latter were discussed in detail.

Enhanced oil-water emulsion separation through coalescence filtration utilizing milkweed fiber: a sustainable paradigm.

Over the past few years, the environment and public safety have suffered due to the detrimental effects of oily industrial effluents. Natural fibers have gained popularity for their affordability, reusability, and effectiveness in separating oil from oily wastewater. Milkweed fibers were characterized using FTIR (Fourier transform infrared spectroscopy), SEM (scanning electron microscopy), and contact angle techniques. With four porosities (0.90, 0.92, 0.95, and 0.98), deep bed coalescence filters were built at three different filter bed heights (10 mm, 20 mm, and 30 mm). Using milkweed coalescence filtering technology, a novel oil separation method is described along with a method to calculate oil film thickness following emulsified oily water saturation. By combining a bed height of 30 mm and a porosity of 0.98, a maximum oil separation of 99.73% and an optimized D50 droplet ratio were achieved. Throughout a prolonged operational period lasting 250 min, the filter bed, possessing a depth of 30 mm and a porosity of 98%, exhibited no discernible fouling indications. Following five cycles, the milkweed filter bed measuring 30 mm in depth and featuring a porosity of 98% displayed an impressive oil separation efficiency of 91.5%. This study found that using a milkweed deep bed filter, coalescence filtering effectively removes oil from oily effluent. Furthermore, milkweed is a natural and biodegradable fiber that is easy to dispose of after use and does not harm the environment.

Influence of the Filtration Velocity on the Local Oil Distribution of Oleophilic Coalescence Filter Media

Fibrous nonwoven coalescence filters are commonly utilized in gas-cleaning processes to separate liquid droplets from a gas stream, e.g., oil mists. These filters are mainly composed of micro glass fibers and in some cases, small amounts of synthetic fibers. The shape of the deposited oil on filter fibers of the filter media depends on several factors, including the oil saturation, wettability, roughness, diameter of the fibers and fiber arrangement. The oil deposits can take the form of, e.g., axially symmetric barrel-shaped droplets or larger structures, such as oil sails between adjacent fibers. Understanding the initial state of the coalescence filtration process and the impact of the deposited oil structures on the separation efficiency requires characterizing these structures. X-ray microtomography (µ-CT) and artificial intelligence tools for segmentation can be utilized to visualize, identify and analyze deposited oil structures in the micrometer region. To quantify and compare oil structures formed at three distinct filtration velocities (10, 25 and 40 cm s−1) commonly utilized in industrial applications and one defined oil saturation of oleophilic coalescence filter media, applying X-ray microtomography is the main emphasis of this work. The results indicate that there is no significant influence of the filtration velocity on the local saturation (determined via µ-CT), the number- and volume-based fractions of the identified deposited oil structures on or between adjacent fibers as well as the droplet concentrations and distributions of deposited oil droplets. It is assumed that the structure of the deposited oil formed by coalescence in the filter medium is dominated by the wetting properties of the fibers (surface tension and surface energy) and the saturation, independent of the filtration velocity.

Investigation of the local oil distribution on oleophilic mist filters applying X-ray micro-computed tomography

Nonwoven coalescence filter media are often used in real-world applications to separate droplet aerosols from a gas stream. While mechanisms like droplet deposition, liquid transport and the evolution of the pressure drop are well understood, information on separated liquid structures formed during filtration is scarce. However, this information is important to understand the initial state of the coalescence filtration process, more specifically, the transition from individual deposited droplets to larger oil structures and subsequently the effect of these oil structures on the separation efficiency.In this work, deposited oil structures in the micrometer region of an oleophilic filter medium of different oil loading stages (saturations) are presented for the first time by utilizing micro–computed tomography (μ-CT) and several post-processing steps. The local (determined via μ-CT) and global (determined gravimetrically) porosities as well as local and global saturations are compared and evaluated. While local (Φlocal=0.9575) and global (Φglobal=0.9588) porosities are in good accordance, significant deviations in the saturation are observed. The local saturation fluctuates significantly at the highest investigated oil loading stage. Especially at high global saturations, local areas are completely saturated with oil. Other areas are weakly saturated, which are particularly relevant for the gas flow to pass through the filter media. Furthermore, a novel routine to distinguish between different coalesced oil objects (e.g. oil droplets or oil sails) is developed and presented. It is shown, that the majority of the deposited oil (>80%) is stored in oil sails between adjacent fibers. Oil droplets surrounding fibers are found to be growing in size with increasing oil loading time. In addition, the volume fraction of the deposited oil in the direction of filter thickness shows, that most of the oil is deposited on the upstream side at the first 80% of the filter.

Modeling oil–mist filtration through coalescence filter media in the presence of gravity-induced flows

This paper jointly addresses numerical and physical challenges related to the modeling of coalescence filtration through vertical filter cartridges, put forth in the following papers by Starnoni, M. and C. Manes:• Starnoni, M., & Manes, C. (2021). A multiphase multicomponent flow and transport model for liquid aerosol filtration in coalescence fibrous filters. Separation and Purification Technology, 266, 118574.• Starnoni, M., & Manes, C. (2022). On the interplay between pressure and gravitational forces in coalescence filters. Journal of Aerosol Science, 105953.In particular, it provides the multidimensional extension of the one-dimensional model presented in Starnoni and Manes (2021) to the general case where internal gravity-induced flows play an effective role. Key to the implementation is the decomposition of the moving oil volumes along the different directions proportionally to the dimensionless number SM, a new quantity introduced in Starnoni and Manes (2022) describing the interplay between the forces dictating the oil dynamics. To validate the implementation, an experimental setup available in the literature is replicated, showing very good agreement in terms of all relevant quantities, namely saturation, pressure drop and drainage volumes. A generalized ‘Network, Channel and Film’ model is also formulated, which extends the scope of application of the original Jump and Channel model by Kampa et al. (2014) to filters with coarser fibrous microstructure. In particular, the existence of an additional flow mechanism termed ‘network’ is postulated, which is characterized by a complex multidimensional oil patterns configuration spanning the entire filter’s height and evolving, in a macroscopic sense, along the direction identified by SM.

Media coalescence filtration as a novel method for pretreatment of refinery wastewater

Refinery wastewater is not only large in volume but also complex in composition. It requires multi-stage treatment to achieve discharge or reuse targets. Pretreatment plays an important role as a front-end treatment. In conventional pretreatment technologies, petroleum removal and suspension removal are carried out through inclined plate settlement combined with two-stage air flotation. This poses problems such as high energy and chemical consumption and large outputs of hazardous waste. In this study, a short-process physical pretreatment technology is designed in conjunction with combined fiber coalescer (CFC) and granular media filter (GMF). The results of the pilot test show that with no added chemicals, the treated refinery wastewater is clear and translucent, and the petroleum and suspended solids content could be reduced to less than 20 mg/L, which falls within the acceptable range of the biochemical influent index. This process could greatly reduce the generation and emission of scum and volatile organic compounds (VOCs) and provides a new technique for the low-carbon pretreatment of refinery wastewater.

Preparation of combined oleophobic fibre filters with pore size gradients and their high-efficiency removal of oil mist droplets

Pore size is a key factor that influences the removal of oil mist droplets by a coalescence filter. However, the influence of the pore size distribution among different filter layers on filtration performance has not been thoroughly explored. In this work, a method for preparing glass fibre filtration media with a specific structure as well as good oleophobic properties and mechanical strength was developed by combining the wet-spinning method and immersive modification. The prepared single-layer media were combined in various three-layer structures with increasing pore size gradient, decreasing pore size gradient and uniform pore size distribution. The trends in the pressure drop and droplet penetration ratio of the resulting combined filters over time were analysed systematically. The results demonstrated that in the combined filter, the first layer material had a significant influence on the filtration process and steady-state performance. Throughout filtration, the pressure drop of the filter with a decreasing pore size gradient had layering characteristics, which was conducive to capturing droplets within different particle size intervals in different stages and thereby relieving the operational pressure on different layers. Hence, the filter with a decreasing pore size gradient had a relatively low steady-state pressure drop and the best comprehensive filtration performance for submicron droplets. However, the filter with an increasing pore size gradient had the highest separation efficiency for submicron droplets and its concentration of steady-state downstream droplets was 19% lower than that in the filter composed of three layers of minimum pore size materials. The proposed mechanism by which the filter with the increasing pore size gradient improved the coalescence filtration efficiency for submicron droplets was based on capillary theory.

Additively manufactured multiplexed inertial coalescence filters

Multiphase flows often pose a significant challenge to the efficient and reliable design of thermofluidic systems. This paper describes multiplexed inertial coalescence filters composed of parallel helical pathways, designed to capture fine droplets (<40 µm) through inertial separation while maintaining a low pressure drop (<400 Pa). Filtration efficiencies for 7 µm and 30 µm droplets were characterized for varying flow conditions, with complete capture observed above a threshold flow rate. Models for filtration efficiency and pressure drop were developed and validated against experimental results to allow system design and optimization, enabled by the tunable additive manufacturing approach used to fabricate the filters. Filter quality factor was computed for varying droplet sizes, showcasing exceptional quality factor when compared to state-of-the-art filters documented in the literature. This multiplexed inertial coalescence filtration approach could find use in dehumidification systems, fog harvesting, chemical reactors, and microgravity droplet capture.

Identification of Deposited Oil Structures on Thin Porous Oil Mist Filter Media Applying µ-CT Imaging Technique

The identification of microscale oil structures formed from deposited oil droplets on the filter front face of a coalescence filter medium is essential to understand the initial state of the coalescence filtration process. Using µ-CT imaging and a deep learning tool for segmentation, this work presents a novel approach to visualize and identify deposited oil structures as oil droplets on fibers or oil sails between adjacent fibers of different sizes, shapes and orientations. Furthermore, the local and global porosity, saturation and fiber ratios of different fiber material of the oleophilic filter medium was compared and evaluated. Especially the local and global porosity of the filter material showed great accordance. Local and global saturation as well as the fiber ratios on local and global scale had noticeable differences which can mainly be attributed to the small field of view of the µ-CT scan (350 µm on 250 µm) or the minimal resolution of approximately 1 µm. Finally, fiber diameters of the investigated filter material were analyzed, showing a good agreement with the manufacturer’s specifications. The analytical approach to visualize and analyze the deposited oil structures was the main emphasis of this work.

Optical investigations into wetted commercial coalescence filter using 3D micro-computer-tomography

Coalescence filtration is used in a variety of process industries to remove liquid mists from gas flows. Non–woven materials are widely used due to their high separation efficiency and low cost. The operating conditions of these filters are dependent on the internal structure of the filter material. Inhomogeneities, arising from the manufacturing process, not only affect the flow of air through the filter but also determine which areas are preferentially wetted, and in turn modify the airflow through the blocked regions.This study uses three–dimensional (3D) micro–computer–tomography imaging to analyze, in a non–destructive manner, the internal structure of a commercial coalescence filter, made of oleophilic glass fibres in dry and wetted state. Using image processing software, a post–processing routine is developed to identify filter parameters based on coarse spatially–resolved images. Afterwards, the dry–filter images are compared with wetted–filter images to establish that inhomogeneous local solidity is responsible for inhomogeneous wetting in an oleophilic filter.

Oil separation from oil in water emulsion by coalescence filtration using kapok fibre

ABSTRACT In this study, the stable emulsion of engine oil in water of concentration 10% was prepared using a non-ionic surfactant. Kapok fibres were used as filter beds to separate oil from the oil–water emulsion. The surface morphology of fibres was investigated using Scanning Electron Microscope (SEM) analysis and chemical bond analysis of fibres done using Fourier transform infrared (FTIR). Kapok filter beds were prepared with three different bed heights 10, 20 and 30 mm each with four different porosities 0.90, 0.92, 0.95 and 0.98 for preparing the coalescence filter. The oil–water emulsion (influent) was pumped into the filtration column and the coalesced oil droplets, water, as well as un-coalesced oil droplets, especially the finer oil droplets, were collected as effluent. Oil separation efficiency was evaluated in terms of change in droplet size (D50) and oil concentration from influent to effluent. With increasing porosity and bed height, apart from porosity of 0.92, the separation efficiency increases. Increasing the bed heights at lower porosities does not improve the efficiency of the process. A combination of 0.98 porosity and a bed height of 30 mm provided the highest filtration performance in terms of oil separation efficiency and D50 droplet ratio. At 0.98 porosity, increasing the bed height from 10 mm to 30 mm resulted in a D50 droplet ratio of 0.25–0.14, representing a significant decrease in droplet size in the effluent and therefore an increase in oil separation efficiency from 91.3% to 99.63%.

Fibrous coalescence filtration in treating oily wastewater: A review

Many industries discharge oil-in-water emulsion in the waste stream, often above the permissible limit causing serious environmental hazards. Porous media such as membrane and coalescence beds are employed to treat oily wastewater. A coalescence bed filter consists of either fibrous or granular packing and is used for removing larger oil droplets less than 100 μm from secondary emulsions. Fibrous media have higher porosities and specific surface areas than coarse granular media and hence give higher oil removal efficiency. To design an efficient fibrous coalescence bed filter for treating industrial discharge, understanding the mechanism of oil separation is important. This involves the surface wettability of fibers relating to surface chemistry and roughness. Further, fiber diameter, filter bed’s height, porosity, and pore size in relation to the oil droplet size and throughput and influent oil concentration are interactive parameters that affect the efficiency of coalescence. The performance of coalescence filtration is evaluated by analyzing the oil concentrations and D50 droplet sizes in the influent and effluent. In this article, the above-mentioned subjects are comprehensively reviewed from the reported research works, which highlights the complex nature of fibrous coalescence filtration.

Numerical investigation of coalescence filtration: Multiphase flow through fibrous structures

Coalescence filtration is a mechanical filtration process, which is used for the removal of liquid droplets from a gas stream. A numerical model is proposed using an Euler-Euler multi-phase formulation to model the macro-scale filtration process. The model is implemented for a 2D case using the commercial CFD software ANSYS© CFX. The capture of the dispersed phase is modelled using the penetration approach for single fiber capture efficiency which is extrapolated to multiple fibers, resulting in an exponentially-decaying interpolation scheme. To model the flow of gas through the dry filter, an empirical pressure-drop correlation is developed based on fiber-scale flow simulations for varied flow parameters. For the fiber-scale simulations, three-dimensional fibers are generated with random position, thickness and orientation using algorithms developed in house, ensuring that all fibers remain in the filter bed and that the fibers do not intersect. The correlation is extended to a wet filter by adapting the correlation parameters, with the assumption that the oil completely wets the fibers and results in a homogeneous increase in net packing density. The local velocity of the fiber-wetting oil is estimated analytically using the local oil saturation and gas velocity using a thin-film approach, taking into account the effect of the mean fiber orientation (determined using three-dimensional micro-CT scans) on the absorbed oil residence time in the filter bed. An analytical model is used to approximate the thickness of the oil film formed at the back of the filter surface and the pressure drop caused by the gas flow through it.Both steady-state and transient simulations are run and validated against measurement data provided by the Institute for Energy and Environmental Technology (IUTA), Germany. The steady-state pressure drop and oil-saturation predictions from the simulations follow the same trend as the experimental data albeit showing a stronger influence on air flow velocity. The transient simulations show the expected three-stage process with an initial gradual increase of the pressure drop, followed by a drastic increase, and finally, an approach to an equilibrium state.

Prediction of residual saturation and pressure drop during coalescence filtration using dynamic pore network model

Microstructure optimization of mist and dust filter media used in many applications including respirators, lubricated machining, compressors and engine crankcases is increasingly becoming reliant on computational simulations of dry and wet filtration processes. Computational fluid dynamics (CFD) based full-morphology simulations with advanced interface capturing techniques offer unique advantages in enabling accurate characterization of the pore-scale transport processes, yet demand substantial computational effort especially for fine filters. In this work, a dynamic pore network modeling (DPNM) framework is developed and evaluated for the prediction of residual saturation and pressure drop during mist filtration, and applied to an oil-air system. Two nonwoven filters (stainless steel and polyester) with different fibre diameters and packing densities are considered. The pore network is extracted using a watershed-based algorithm, from realistic virtual filter geometries (Abishek et al., 2017), and validated for macroscopic properties such as packing density. The validity of the steady PNM model is assessed by comparison of the dry (air) pressure drop against CFD and experimental data in the literature. The validity of the DPNM model is assessed by comparison of multiphase pressure drops and residual saturation against micro-CT and gravimetric experiments carried out in this work, as well as pore-scale CFD simulations using the volume-of-fluid model. It is found that the dynamic pore network model is able to successfully predict the critical properties of the filtration process such as saturation (local and total) and pressure drop to within 15% of experiment and CFD. Considering that the overall computational cost of DPNM is about 2–3 orders of magnitude lower than fully resolved CFD, DPNM stands as a potential tool for the early-stage design and optimization of mist filtration media.

Representative domain size for the simulation of coalescence filtration in nonwoven and foam media

Abstract Pore-scale filtration simulations require high spatio-temporal resolutions and significant computational effort, hence, keeping the domain size to a minimum is desirable. Previous studies have considered domains based on Brinkman length, or are limited by computing power, and little information is available for conditions involving high fluid saturation – typical of steady state mist filtration. In this study, simulations are performed to characterize the effect of domain size on pressure drop, residual saturation, liquid film thickness and interfacial area concentration, using virtual nonwoven and foam filters with similar micro-structural properties. Further, experiments using micro-CT are performed to validate the present computational simulations. It is found that two phase flow through filters are more sensitive to local geometric variations or mesh resolution in the porous media than single phase flow. Statistical uncertainties in the steady state quantities of less than ± 10% can be expected to cope with the increase in computing power required for practical mesh sizes. A computational domain size of about 50–100 × d (where d is the strut or fibre diameter) was found to be required for CFD for the operating conditions considered.

Improved design of two-stage filter cartridges for high sulfur natural gas purification

Abstract Two-stage filtration units including pre-filtration and coalescence filtration play an important role in the quality control of high-sulfur natural gas, because micron-sized particles can significantly influence the gas sweetening process. Filtration failure usually occurs in operation, because of the mismatch between the two types of filter cartridge in the filtration unit. To overcome this drawback, the performance of the coalescing filter cartridge must be improved and a matching pre-filter cartridge need to be designed. One approach for accomplishing this is to spray an atomizing adhesive onto the fibrous layers of the filter cartridge to enhance its integrated filtration performance. In this study, a pleated protective layer and a hydrophobic drainage layer were designed for the coalescing filter cartridge to prolong the operation cycle and quickly drain the captured liquid. Testing results showed that the new-type of coalescing filter cartridge exhibited 75% decrease in its steady pressure drop over that of the original filter and the filtration efficiency reached 99.99% for liquid droplets with diameter greater than 0.3 µm. The structure of pre-filter cartridge was also redesigned to match the performance of the new coalescing filter cartridge. As a result, the operational loading of the improved two-stage filter cartridges achieved the desired performance. Field testing of the new filter combination showed that the acid gas filtration unit produced a more reasonable pressure drop and a higher rate of captured liquid than with the conventional filter.